Study reveals dramatic decline in historic sargassum populations, with WHOI scientists helping unravel basin-scale changes

Woods Hole, Mass. (Dec 5, 2025) -- A new study published this week in Nature Geoscience has uncovered a dramatic decline in long-standing populations of the seaweed Sargassum in the North Atlantic’s Sargasso Sea and Gulf of Mexico, and this shift could have significant ecological consequences. While some regions of the tropical Atlantic have seen increases in Sargassum over the past decade, researchers found that populations in the northern Sargasso Sea have plummeted since 2015.

Scientists from Woods Hole Oceanographic Institution (WHOI) played a role in the analysis, which was led by the University of South Florida’s College of Marine Science and included partners from Sea Education Association (SEA), Eckerd College, and Florida Atlantic University. The findings point to warming ocean temperatures, shifting nutrient regimes, and changing circulation patterns as likely contributors to this emerging basin-scale redistribution of Sargassum.

Sargassum, which forms extensive floating mats in the open ocean, is a foundation species supporting rich biodiversity, including juvenile sea turtles, crabs, fish, shrimp, and seabirds. But when Sargassum washes ashore in excess, it can create environmental and economic disruptions, costing coastal regions millions of dollars each year in cleanup efforts.  Human health can also be affected by hydrogen sulfide gas emanating from the decaying seaweed.

In the new study, researchers paired long-term satellite observations with field measurements and historical datasets to track changes in Sargassum biomass and seasonality. WHOI oceanographer Dennis McGillicuddy, a co-author on the paper, contributed expertise on the ocean circulation and biophysical processes that govern how Sargassum is transported and sustained across the Atlantic basin.

“What makes this problem so complex is that it involves physics, biology, and chemistry of the ocean on scales ranging from the Sargassum mats themselves all the way up to ocean basins,” said Dennis McGillicuddy, senior scientist at Woods Hole Oceanographic Institution and Director of the Woods Hole Center for Oceans and Human Health. “What we are seeing now suggests a shift in the basin-wide dynamics that have historically supported healthy Sargassum populations in the Gulf of Mexico and Sargasso Sea.”

The study showed that record-breaking marine heat waves in the Gulf of Mexico may be weakening Gulf populations before they reach the Sargasso Sea. This diminished supply, combined with warming conditions in the open ocean, may be reducing the resilience of northern Sargasso Sea populations.

WHOI helped analyze how these changing environmental conditions affect Sargassum transport pathways, including alterations to seasonal patterns. Historically, peak sargassum in the Sargasso Sea occurred in late fall and early winter; the new findings show this peak has shifted to summer, a hallmark of broader ecological change.

Looking ahead, researchers will continue investigating how warming trends, nutrient availability, and large-scale circulation changes interact to shape Sargassum dynamics, and whether increasing biomass from the Great Atlantic Sargassum Belt could further alter the balance of Sargassum populations in the Gulf and Sargasso Sea.

A version of this article appeared on the USF College of Marine Science news site.

WHOI gratefully acknowledges philanthropic support of T. and E. Bowler and support of the Woods Hole Center for Oceans and Human Health from the National Institute of Environmental Health Sciences (2P01ES028938) and the National Science Foundation (OCE-2418297).

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About Woods Hole Oceanographic Institution

The Woods Hole Oceanographic Institution (WHOI) is a private, non-profit organization on Cape Cod, Massachusetts, dedicated to marine research, engineering, and higher education. Established in 1930, its primary mission is to understand the ocean and its interaction with the Earth as a whole, and to communicate an understanding of the ocean’s role in the changing global environment. WHOI’s pioneering discoveries stem from an ideal combination of science and engineering—one that has made it one of the most trusted and technically advanced leaders in basic and applied ocean research and exploration anywhere. WHOI is known for its multidisciplinary approach, superior ship operations, and unparalleled deep-sea robotics capabilities. We play a leading role in ocean observation and operate the most extensive suite of data-gathering platforms in the world. Top scientists, engineers, and students collaborate on more than 800 concurrent projects worldwide—both above and below the waves—pushing the boundaries of knowledge and possibility. For more information, please visit www.whoi.edu